The present invention relates to an engine type forklift employing a hydraulic driving system.
Heretofore, main power transmission devices for general engine type forklifts have been of a clutch type and a torque converter type; besides this, there is also an engine type forklift employing a hydraulic driving system. This engine type forklift employing a hydraulic driving system is available as a 1-pump 1-motor type, a 1-pump 2-motor type, etc., of which characteristics include high efficiency and unnecessitation of front differentials. As for the turning system, rear wheels are turned for direction change.
Further, conventional general forklifts have a center of gravity positioned at a higher level than in the case of automobiles and have a cradle mechanism, with the result that they have so low turning stability that they could overturn if they make a sharp turn at high speed. As an example of countermeasures therefor, the amount of cradling is controlled according to speed. Further, since the overturning during a turn occurs more often when the traveling speed is high or when the turn is sharp, it is advisable to reduce the traveling speed during the turn; however, the operator""s will (the amount by which the accelerator pedal is pressed) is involved in this case, so that mistakes in operation tend to arise. Accordingly, there has been provided a system for automatically controlling the traveling speed, which feeds back the turning angle of the steering wheel and uses an electronic governor or the like to change the engine rpm (revolution per minute).
However, according to the conventional arrangement described above, the use of a pump shared by the right and left front wheels that are driving wheels results in poor maneuverability and in an increased turning radius. Further, according to a system for controlling the traveling speed during a turn by changing the engine rpm, the mechanism becomes complicated and the traveling speed cannot be optionally set.
Further, the brake system is of a type in which a drum brake is attached to the front hub as in a usual forklift, presenting problems from the standpoint of cost and space. Further, it cannot be used in the form in which hydraulic motors are attached directly to the front wheels.
Accordingly, a first object of the present invention is to provide a hydraulically driven forklift employing a 2-pump 2-motor type, which makes it possible to improve maneuverability and to reduce the turning radius.
A second object of the invention is to provide a hydraulically driven forklift capable of using a hydraulic brake provided by a hydraulic drive system as a service brake.
In order to achieve the first object mentioned above, the invention provides a hydraulically driven forklift having a vehicle body provided with a pair of right and left front wheels and a pair of right and left rear wheels, and a mast and forks disposed at the front end of the vehicle body, characterized in that the front wheels are respectively operably connected to the drive shafts of hydraulic motors attached to the vehicle body, the vehicle body is provided with a plurality of hydraulic pumps driven by an engine, each hydraulic pump corresponding to and being connected to one of the hydraulic motors, and the rear wheels are provided to be turnable around vertical axes with respect to the vehicle body.
According to the above arrangement of the invention, the control of speed in longitudinal travel can be effected by switching the directions of oil flow from the both hydraulic pumps by a change lever, changing the direction of rotation of each hydraulic motor, and controlling the rpm of the engine and the flow rates of oil from the hydraulic pumps by an accelerator pedal, thereby changing the rpm""s of the hydraulic motors. And the control of turning is made possible by controlling, based on the turning angle of a steering wheel or of the turning angles of the rear wheels, the respective rotation speeds (equal or different) or the respective directions of rotation (same or opposite) of the right and left front wheels.
Further, it is possible to improve maneuverability and reduce the turning radius by employing a 2-pump 2-motor type hydraulic drive system (HST system) as the drive system for the forklift, and separately controlling the right and left front wheels which are drive wheels. And it is possible to simplify the power transmission system and increase the degree of flexibility of layout by respectively attaching the front wheels directly to the hydraulic motors attached to the vehicle body. Further, high efficiency and unnecessitation of a front differential, which are the characteristics of the hydraulic driving system, can be expected; and besides, low fuel consumption due to optimum control of the engine can be expected.
A preferred embodiment of the invention is characterized in that the change of traveling speed during a turn is made by controlling the rpm""s of the hydraulic motors according to the turning angle of the steering wheel.
According to this preferred embodiment, the traveling speed during a turn can be automatically changed by controlling the rpm""s of the hydraulic motors according to the turning angle of the steering wheel without changing the rpm of the engine. And the arrangement can be simplified and the traveling speed during the turn can be optionally set independently of the accelerator pedal.
The preferred embodiment of the invention is also characterized in that the change of traveling speed during the turn can be made by controlling the rpm""s of the hydraulic motors according to the turning angle of the rear wheels.
According to this preferred embodiment, the traveling speed during the turn can be automatically changed by controlling the rpm""s of the hydraulic motors according to the turning angle of the rear wheels without changing the rpm of the engine. And the arrangement can be simplified and the traveling speed during the turn can be optionally set independently of the accelerator pedal.
To achieve the second object mentioned earlier, another embodiment of the invention is characterized in that the hydraulic pumps are of an electrically controlled type arranged to control swash plate angles based on a travel instruction signal from a controller, the arrangement being such that the travel instruction signal is emitted from the controller into the hydraulic pumps by feeding into the controller a detection signal from a detector for detecting an amount of travel of a brake pedal.
According to this embodiment, when the brake pedal is pressed, the amount of travel thereof is detected by the detector and the detection signal is fed into the controller, whereupon a travel instruction signal is emitted from the controller according to the detection signal into the hydraulic pumps, thus controlling the swash plate angles of the hydraulic pumps. That is, braking can be effected by controlling the speed at which the swash plate angles of the hydraulic pumps are returned to 0xc2x0 according to the amount of travel of the brake pedal. On this occasion, it is arranged that the swash plate angles of the hydraulic pumps become 0xc2x0 a little before a stroke end of the brake pedal, and at the stroke end, parking brakes built in the hydraulic motors can also be actuated.
Therefore, hydraulic brakes provided by the hydraulic drive system can be used effectively as the service brake, which is suitable from the viewpoint of cost and space. This arrangement is also applicable to a type where the hydraulic motors are directly attached to the front wheels. Further, as in ordinary torque converter type forklifts, inching operation can also be effected by using the brake pedal.